i) Field of the Invention
The present invention relates to a magneto-optical information reproducing apparatus for reproducing information recorded in a recording medium in which a groove and a land are alternately formed and the information is recorded both in the groove and land by applying a magnetic field to the recording medium and irradiating the recording medium with light.
ii) Description of Related Art
An optical recording medium has heretofore been used broadly as a recording medium by which a sound signal and an image signal can be recorded/reproduced. Particularly, a magneto-optical recording medium has been noted as a high-density recording medium which can rewrite information, and intensively researched/developed. Moreover, a magneto-optical information reproducing apparatus for reproducing the information recorded in the magneto-optical recording medium has also been intensively researched/developed.
The magneto-optical recording medium generally has a disk form, and is provided with a spiral or concentric track, and the information is recorded in the track. Moreover, in order to enhance a recording density of the magneto-optical recording medium, two techniques of shortening a track pitch and of enhancing a linear recording density are considered. Either technique can be realized by shortening a wavelength of a semiconductor laser for use in recording/reproducing, but it takes some more time to realize a semiconductor laser in an inexpensive manner which continuously oscillates a short wavelength of green or blue at a room temperature steadily for a long time.
In this situation, a technique is sought which uses a semiconductor laser with the current wavelength to largely improve the linear recording density, such as a magnetic super resolution (hereinafter referred to as MSR) for applying a reproducing magnetic field to the magneto-optical recording medium to limit a reproducing object to an area smaller than a laser spot.
Moreover, as the technique of shortening the track pitch, a technique of land and groove recording has been proposed in which the information is recorded both in a furrow-like groove and a ridge-like land alternately disposed on the recording medium. In this land and groove recording, both the groove and the land are used as the track. Therefore, in the land and groove recording, as compared with a technique of using only one of the groove and the land as the track, the track pitch simply becomes xc2xd. When the linear recording density is the same, the recording density can be doubled. Therefore, this technique is considerably important in realizing a high-density recording.
Additionally, in the land recording in which the information is recorded only in the land, the groove exists between adjacent lands, there is a gap between the lands in which the information is recorded, and occurrence of a crosstalk in which the information recorded in the adjacent land is mixed into reproducing information is therefore inhibited.
FIG. 1 is a diagram showing that the information recorded by the land recording is read.
Here, a groove 2 narrower than a land 1 is formed between the lands 1, and a recording mark 3 in accordance with the information is formed only on the land 1. Specifically, only the land 1 is used as the track, and a track pitch is, for example, about 0.9 xcexcm. Moreover, the information is recorded by a mark space row constituted of the recording marks 3 and spaces 3xe2x80x2 between the marks.
Moreover, FIG. 1 shows that the recording marks 3 are read by the aforementioned MSR. In the recording medium for the MSR system, the recording mark 3 and space 3xe2x80x2 are in magnetization states different in magnetization direction from each other, the magnetization state of the recording mark 3 is hereinafter referred to as a recording state, and the magnetization state of the space 3xe2x80x2 is hereinafter referred to as a clear state. Moreover, the information is read by recognizing these magnetization states, but for the sake of convenience of description, the recognition of the magnetization state is sometimes referred to as xe2x80x9creading of the recording markxe2x80x9d.
During the reading of the recording mark 3, a laser spot 4 is tracked on the land (track) 1, and relatively moved in a direction of an arrow F with respect to the land 1 so that the recording mark 3 is read. In this case, a reproducing magnetic field is applied to an area held between a dotted line L1 and a dotted line L2. When the reproducing magnetic field and heat by the laser spot 4 both act on a material constituting the magneto-optical recording medium, a front mask 4a and rear mask 4b are formed to hide the presence of the recording mark 3. As a result, a range 4c in which the recording mark 3 can be read is limited to a range smaller than an irradiation range of the laser spot 4, and even the recording marks 3 recorded within a laser spot diameter can be read one by one. Moreover, by recognizing the recording mark 3 and space 3xe2x80x2 constituting the mark space row, the information is reproduced.
As described above, since the groove 2 is formed between the lands 1, the recording mark 3 on a land 1b adjacent to a land 1a in which the laser spot 4 is tracked fails to enter the range 4c in which the recording mark can be read, and occurrence of the crosstalk is inhibited.
Similarly, in groove recording in which the information is recorded only in the groove, the land exists between adjacent grooves, there is a gap between the grooves in which the information is recorded, and occurrence of the crosstalk in which the information recorded in the adjacent groove is mixed into the reproducing information is inhibited. On the other hand, in the land and groove recording in which the information is recorded both in the land and the groove, no gap exists between the tracks, the occurrence of the crosstalk from the groove adjacent to the land or from the land adjacent to the groove cannot be avoided, and a large influence is therefore exerted to information reproducing ability.
FIG. 2 is a diagram showing that the information recorded by the land and groove recording is read.
Here, the land 1 and groove 2 with the same width are alternately formed, and the mark space row constituted of the recording marks 3 and spaces 3xe2x80x2 in accordance with the information is formed both on the land 1 and groove 2. Specifically, both the land 1 and the groove 2 are used as the tracks, and the track pitch is a narrow pitch, for example, of about 0.6 xcexcm.
FIG. 2 also shows that the recording mark 3 is read by the aforementioned MSR, and here shows, as one example, that the recording mark 3 on the land 1 is read. By irradiating the land 1 as a reading object track with the laser spot 4, relatively moving the laser spot 4 in the direction of the arrow F, and applying the reproducing magnetic field to the area held between the dotted lines L1 and L2, the front mask 4a and rear mask 4b are formed.
As described above, since the recording mark 3 is written both in the land 1 and groove 2, the recording mark readable range 4c also reaches the recording mark 3 on the groove 2 adjacent to the land 1 in which the laser spot 4 is tracked, and the crosstalk occurs.
For example, in Japanese Patent Application Laid-Open No. 7357/1996, a technique of appropriately selecting a groove depth and reducing the crosstalk from the land or the groove is proposed. However, in the groove with the selected depth, a carrier level corresponding to original reproducing information is lowered, and there is also a problem that a push/pull signal level for use as a tracking error signal is similarly lowered. Moreover, it has been already reported that the crosstalk reduction effect obtained by selecting the groove depth is easily collapsed by fluctuation of Kerr ellipticity, and focus error, spherical surface aberration, and the like of an objective lens.
Moreover, in Japanese Patent Application Laid-Open No. 153221/1997, a technique of reversing polarities of signals recorded in the land and groove to reduce the crosstalk is proposed. However, when the signal polarities of the land and groove are reversed in magneto-optical recording, noise is generated in a boundary of the land and groove, which raises a problem more than the crosstalk.
In consideration of the aforementioned situation, an object of the present invention is to provide a magneto-optical information reproducing apparatus in which information reproducing can be performed at a low crosstalk level.
To achieve the aforementioned object, according to the present invention, there is provided a magneto-optical information reproducing apparatus for reproducing information recorded in a recording medium in which a groove and a land are alternately formed and the information is recorded both in the groove and land, and for reproducing the information by applying a magnetic field to the recording medium and irradiating the recording medium with a light, the magneto-optical information reproducing apparatus comprising: a magnetic field generating section for generating the magnetic field to be applied to the recording medium; and
an adjusting section for adjusting an intensity of the magnetic field generated by the magnetic field generating section with respect to the land and groove so that with respect to one of the land and the groove in which reproducing object information is recorded, a level of a crosstalk attributed to the information recorded in the other one of the land and the groove is lowered.
Here, the xe2x80x9ccrosstalk levelxe2x80x9d may be a level obtained by noticing only one of the crosstalks attributed to the land and groove, or a level obtained by generalizing the crosstalks attributed to the land and groove.
According to the magneto-optical information reproducing apparatus of the present invention, since the intensity of the magnetic field applied to the recording medium during information reproducing is adjusted so that the crosstalk level is lowered with respect to the land and groove, the information reproducing in a low crosstalk level is possible.
In the magneto-optical information reproducing apparatus of the present invention, the adjusting section preferably adjusts the intensity of the magnetic field in accordance with the intensity of the light radiated to the recording medium so that the crosstalk level is lowered.
The crosstalk level to the magnetic field intensity differs in accordance with the intensity of the light radiated to the recording medium during reproducing. Moreover, the light intensity is adjusted in accordance with environment temperature during reproducing. Therefore, by adjusting the magnetic field intensity in accordance with the intensity of the radiated light, the information reproducing in the low crosstalk level is assured.
Moreover, preferably the magneto-optical information reproducing apparatus of the present invention xe2x80x9cfurther comprises a relation data preparing section for reproducing predetermined information with a plurality of magnetic field intensities and a plurality of irradiation light quantities and measuring a crosstalk level to prepare relation data indicating a relation among the magnetic field intensity, irradiation light quantity and crosstalk level based on a measurement result, and
the adjusting section adjusts the intensity of the magnetic field based on the relation data prepared by the relation data preparing section.xe2x80x9d
Generally, the relation among the magnetic field intensity, irradiation light quantity, and crosstalk level depends on a material type of the recording medium. For the recording medium on the market, however, even when the information reproducing is possible by the same magneto-optical information reproducing apparatus, various constituting materials are used in accordance with makers and product types, and it is impossible to store the relation among the magnetic field intensity, irradiation light quantity and crosstalk level beforehand with respect to all disk material types.
According to the magneto-optical information reproducing apparatus provided with the relation data preparing section, the relation data indicating the relation among the magnetic field intensity, irradiation light quantity and crosstalk level is prepared based on measurement, and the magnetic field intensity is adjusted based on the relation data. Therefore, even when individual recording medium materials are unknown, the crosstalk can be depressed to a low level.
Furthermore, in the magneto-optical information reproducing apparatus of the present invention, the relation data preparing section preferably records a period signal for the land and a period signal for the groove different in frequency from each other in the land and groove of the recording medium, regenerates the recorded period signals with the plurality of magnetic field intensities and the plurality of irradiation light quantities and measures the crosstalk level to prepare the relation data based on a measurement result.
According to the preferably constituted magneto-optical information reproducing apparatus, since the period signal for the land and period signal for the groove different in frequency from each other are recorded in the recording medium, and these period signals are used as signals for crosstalk measurement, it is easy to separate original signal components from crosstalk components, and the crosstalk measurement is facilitated.